Automatic prf selection to optimize range and doppler visibility in radar tracking

ABSTRACT

A radar system is disclosed which uses an established track predicted range, Rp, and radial velocity, Rp, of a target to select a frequency, F, and PRF for optimum range and doppler visibility in tracking. The procedure is to tentatively select a frequency not subject to interference; determine the highest possible band of PRF&#39;&#39;s allowable between adjacent velocity blind regions computed from the equation

United States Patent [1 1 Parke et al.

[4 1 Dec. 31, 1974 i 1 AUTOMATIC PRF SELECTION TO OPTIMIZE RANGE AND DOPPLER VISIBILITY IN RADAR TRACKING [75] Inventors: Donald P. Parke, Anaheim; Willis M. Priester, Garden Grove; Stuart P. Scheidler, Anaheim; Gerald M. Goldberg, Placentia, all of Calif.

[73] Assignee: Hughes Aircraft Company, Culver City, Calif.

[22] Filed: Feb. 5, 1973 [2]] Appl. No.: 329,763

[52] U.S. Cl. 343/75, 343/5 DP, 343/77, 343/ l7.l PF {51] Int. Cl. G015 9/10 [58] Field of Search 343/5 DP, 7.5, 7.7, l7.1 PF

[56] References Cited UNITED STATES PATENTS 3,066,289 ll/l962 Elbinger 343/77 3,258,769 6/l966 Forestier 343/75 3,588,898 6/l97l Watters et al. 343/77 3,787,85l l/l974 Hughes 343/7.7

(57] ABSTRACT A radar system is disclosed which uses an established track predicted range, R and radial velocity, R, of a target to select a frequency, F, and PRF for optimum range and doppler visibility in tracking. The procedure is to tentatively select a frequency not subject to interference; determine the highest possible band of PRFs allowable between adjacent velocity blind regions computed from the equation (PRF) 2R,,F/cn 1 A(PRF) where A(PRF) is a known half width of velocity blind regions from their centers, c is the speed of light, and n=l,2,3 determine the highest PRF within the allowable band which places the target outside of range blind zones between R" and R,,+8. where 6 is the width of blind zones and R, for the nth blind zone is determined from the equation Where n=l,2,3 A preliminary check determines 12 Claims, 21 Drawing Figures q! pl. m. m-

T 9- wain I257 II 1! i rm Bum I F sunl cmm M l l l l l l l l l I l w l l momma! T W t, .s l

PAIENTED DECS I I974 SHEET 01 0F 19 L I I I I I I I I I I I I Ill-I'll- Phased array antenna Phased array antanna mm .mm Sp mum 8 2m m r I. a IIIIIII m g m n .I a .m m m T C li II. IIIII a 1m J U y m r m X m w W I E V T v r w m r .m 8 8 m m .lm m D. I m mm mm m D Rb APw m. w m r .w R m |II|I|I|. 1 u A 3 m )I 1 VM Va 8 WW I 8 0m 0m Transmitter Antenna Switching IZITEHTEU GU13 1 I874 SHEET UEUF 19 (3. 2C]. (:1 (1225 MHz) I 1222 1227 MHz 23, '4

I r o Bandpass Integrator A/ D Conv. filter 1 I J CP c2l1230 MHZ) I I T 0 Clear 288 MHZ $33M Integrator AID Com. 3 channel Radar senslng input Q2/4232 MHz I I data to from I rec. aw

CP buffer C32 (1375 MHZ) r v j 0 3 Integrator MD Conv. falter 0 1372 1377 MHz FJ Clear C t onver Sample N F-\ G I Radar 16 MHz m read gate I CP 1 0 I 1 o l 1 Q FFZ FF 4 4 l i r 7 G2 25 Fig 2b 133 13c I I 1 13b I Fargm FF] Power 7- 1st COHV. miter I a tenna I I switching Clear From 1st I I L. L0. in I 83mph exciter -II (Fig. 3) I 13 Convert To other stages of receiver :MHz) 1 3 t 5 3 7 :1

PAlENlEunana 1 I974 SHEU 03 0F 19 2 Gate 31,uS Gate 28 26 l r 29 r 1 Pulse r 32 MHz 6 7 expansion G Mlxer 1 line 27 288 M Hz Freq 256 MHz mult. (X8) Freq system 10 MHz 7 divider clock (+2) 5 MHz) B.W. filter From 7 computer G 7 First loml 1 oscillator ixer radar generator data v (Fig. 4) buffer Power amp First local F I 3 oscillator signal to receiver 35 To transmitter Fig. 5.

SECTION I Input parameter R R Track range ri me radial velocity All 56 32 frequencies finished 51 Set F next best frequency 52 5 from ordered frequency table Sort the 32 6 frequencies into preferred order using jam amplitude associated with freq.

PR F 020 I 1 Set F to best frequency (lowest 55 jam amplitude from deer channel sensing) Set no 53- visibility flag Return FATENTED l 1974 3,858,208 SHEET UBUF 19 Fig. 6.

l I Calculate allowable (PR F) x bands for this F I 1562 (PRFl s lPFlFl lPRFl -r' 3125 I l 7 J Calculate 1st l l t m forbldden PRF 38m; Q Hm time Off ss- FX lower bound (PRHAU 3125 74 F PRF) (PFlFl lPRF) IL (PRFl 1562 Calculate 1st forbidden PRF F. lower bound this FX 63 "it t me Yes 3.235 (10! lPFlFl 1562 First time off (PRFl 3125 First time 67 all PHI 140 I 73 first time on (PRFl (PRF)FU FUEHTED 3.858.208

SHEET 07 0F 19 SECTION II (CONTINUED) Fig. 7.

63 (PRHFLK 1562 5 'AL (PRF) (PRF) 72 71 AL AU SECTION III PRF 200 6 Check if H is in blind range zone for this (PRF) 76 (PRF) (PRFAU 78 Calculate 1st 1 blind range zone 01' PRF) 77 ls x 3 x R1 8 81 (10) (PRF PATENTEU 3.858208 SHEET 08 0F 19 SECTION III (CONTINUED) Fig. 8.

Visibility good x 30 Frequency F 83 (PRF) k PRF PRHx X (PRF) Return PATENTEI] BEC3 1 I974 SHEU lUUF 19 Fig. 10.

Processor (optional) Memory (16K) L F. l

Address/data to memory Data from memory lnterprooessor oontrol Input/output Control module Data from memory Ch I 0 g s Address/data to memory Data from memory Channals 4 7 Address/data to memory f onnectrons or Control additional modules Data from memory Address/data to memory Processor (required) no 111 Memory (16K) FATENTED 1 I974 3 858,208

SHEET 110F19 I Real time clock v Interrupt register Flg. 11.

Interrupt network Address generator 122 Address buss selector To memory 1 Control 1 unit Program counter v To memory data buss selector L Shift 120 counter From Timing Command memory counter register 126 To 8 Register corflrol Compare network 11a 1 124 Adder Sense indicators A Register 0 Register mrmanm 1 I914 3 858 208 SHEET 13B? 19 Subtramr From converter 256 Sweep storage Subtractar 1st diff. storage Limiter To coherent integrator FATENTED 1 3 858.298

sum 1n 0F 19 Fig. 14.

Nonooherent Coherent integrator integrator Acc.

I Recombine 3&7 I Acc. I

292 L J o n O b I 296 Output h of F Coefficient m 294 dopples logic processor network I h 285 From radar control unit FHENTEB SE83 1 I974 SHEET 15 0F 19 For next dwell M (mode) From computer radar control F (frequency) Excite, command ID (array ID) Antenna EL (sin elevation angle) v phase A: (sin azimuth angle) computer M, F, )0, EL, Az, p pm: PRF (pulse repetition freq) R d B (burst no.) a

FP (fill pulses) sync 'omzer TH (threshold) R start (range cell start) Received R stop (range cell stop) AF (ground doppler offset) signal processor For this dwell To computer beam return report CL (clutter level) CT (subreport count) J (iam level) Fl (range) F no. (filter no.)

SIC+N (signal/clutter Noise) Signal processor Fig. 150.

RADAR DATA BUFFER FATENTEU (R3 1 I974 PRF R start R stop SHEET IBUF 19 Fig. 15b.

EL AFG R start R stop PRF Bits

(array ID) 00 Fore 10 Aft 2 01 Strbd 11 Port (mode) 0 Search 4 Test 3 1 Verify 5 Sea State 2 Track 3, 6 and 7 not defined (sine of A2 angle) sin .088 i sin 55 12 (sine of EL angle) sin .088 i sin 93 12 (threshold) 5 db 7 20 db 6 (frequency) 0 31 5 (gnd doppler offset) 3 Hz 1 4000 Hz 12 (pulse repetition freq) lPPS 15623125 12 (fill pulsesl 1 pulse 8 13 4 (no. of bursts) 1 burst 0 3 (N 1) 2 (range cell start) 1 RC 0- 5000 13 (range cell stop) 1 RC 0 5000 13 RADAR CONTROL COMMAND FORMAT ['IXTENTEU 3.858.208

SHEET 17 HF 19 2 RCT 3 4 Header Report Header (one per dwell) F (frequency) 0 31 5 Az l sine of A2 angle) sin D88 1 sin 55 12 EL (sine of EL angle) sin .088 i sin 9.8 12

CL (clutter level) 0 31 5 ID (array ID) 00 Fore 10 Aft 2 01 Strbd 11 Port SRCT (sub-report count) 0 4 63 6 M (mode) 0 Search 4 Test 3 1 Verify 5 Sea State 2 Track 3, 6 and 7 not defined ljam level/frequency) I 0 7 3 PRF (pulse repetition frequency) IPPS 1562-3125 12 Report ("N" per dwell) R (ambiguous range) IHC 0 5000 13 S (signal/clutter noise) 2 db 8 r 40 db 4 A (amplilude) 3/8 (lb 0 (lb 9 F (liller number) 0 15 4 Fig. 15c.

BEAM RETURN REPORT FORMAT 

1. In a moving-target-indication (MTI) radar system for tracking targets, said system having a transmitter for transmitting radar pulses at a selected transmitting frequency, F, a receiver for receiving radar return signals, and a signal processor for cancelling all radar return signals except those having a doppler shift due to a moving target, apparatus for automatically selecting a pulse repetition frequency, PRF, within a given range of PRF''s for optimum range and doppler visibility, comprising computer means for selecting said transmitting frequency, computer means for finding a band of PRF''s between velocity blind regions in said range of PRF''s, where a velocity blind region is a small band of PRF''s centered on a PRF which happens to be the same as the doppler frequency of the target being tracked, or a multiple thereof, computer means for finding a particular PRF within said band of PRF''s which does not have a range blind zone spanning the predicted range of a target being tracked, and radar synchronizing means for timing transmission of radar pulses at said particular PRF by said transmitter, each pulse consisting of electromagnetic energy at said transmitting frequency.
 2. Apparatus as defined in claim 1 wherein, said computer means for finding said particular PRF includes means for causing said computer means for finding a band of PRF''s to find another band of PRF''s between velocity blind regions in said range of PRF''s upon failing to find a PRF within said band of PRF''s which does not have a blind zone spanning the predicted range of a target being tracked, and said computer means for finding a band of PRF''s includes means for thereafter causing said computer means for finding said particular PRF to search the new band of PRF''s for said particular PRF, and further includes means for terminating the resulting iterative search for a band of PRF''s and a particular PRF within said band when a particular PRF is found which does not have a blind zone spanning the predicted range of a target being tracked.
 3. Apparatus as defined in claim 2 including means for selecting said frequency, F, from a plurality of predetermined frequencies, comprising clear channel sensing means connected to said receiver for sensing the magnitude of electromagnetic energy received by said receiver in a plurality of frequency channels, each of predetermined bandwidth spanning a different one of said predetermined frequencies, means for converting said magnitude of energy in each channel to a value in digital form, computer means for sorting said values in digital form in order of magnitude, and computer means for selecting a transmitting frequency corresponding to that frequeNcy of a channel having the lowest of said values of sensed electromagnetic energy.
 4. Apparatus as defined in claim 3 wherein, said computer means for selecting a transmitting frequency includes means for selecting a frequency corresponding to that frequency of a channel having the next lowest of said values of sensed electromagnetic energy during the next iteration of an iterative search for a band of PRF''s and a particular PRF within said band upon said computer means for selecting a transmitting frequency, failing to find a particular PRF within said band of PRF''s which does not have a blind zone spanning the predicted range of a target being tracked in any band of PRF''s between velocity blind regions in said range of PRF''s.
 5. Apparatus as defined in claim 4 wherein said computer means for finding a band of PRF''s between velocity blind regions includes means for finding a band of PRF''s by determining blind regions in accordance with the equation (PRF) 2RpF/cn + or -Delta (PRF) where n 1,2,3. . . ,c is the speed of light, and Delta (PRF) is a predetermined value included to allow for half widths of velocity blind regions for a predicted radial velocity, Rp, of a target being tracked and said selected frequency, F, a different velocity blind regions resulting for each value of n, and a different band of PRF''s allowable for tracking falling between successive velocity blind regions.
 6. Apparatus as defined in claim 5 wherein said computer means for finding a particular PRF within said band of PRF''s which does not have a range blind zone spanning the predicted range, Rp, of a trarget being tracked includes means for determining the range blind zone for each PRF within said band of PRF''s until one is found not having a range blind zone spanning said target at said predicted range in accordance with the following equation Rn cn/2(PRF) where n is an integer 1,2,3. . . ,c is the speed of light, and Rn is the beginning of a blind range zone of predetermined width, delta .
 7. Apparatus as defined in claim 6 wherein said means for determining a band of PRF''s by determining blind regions includes means for determining an approximation (PRF) about RPF/(2.94(10)8 + or - Delta F)n where Delta F is a percentage factor to allow for the half width, Delta (PRF), of each velocity blind region.
 8. Apparatus as defined in claim 6 wherein said means for determining the range blind zone for finding range blind zones for each PRF within said band of PRF''s includes means for determining an approximation Rn about 81 (10)3n/(PRF) of the equation Rn cn/2(PRF) and the half width, delta , of each range blind zone is as predetermined.
 9. In a radar system having a transmitter and a receiver for tracking targets, apparatus for automatically selecting a pulse repetition frequency, PRF, at a transmitting frequency to use for the next dwell in updating track data of a selected target, said data including predicted range, Rp, and radial velocity, Rp, and said PRF being selected to assure target visibility in range and doppler shift of radar return signals in order to be able to update said target data, said apparatus comprising computer means for calculating possible bands of PRF''s between adjacent velocity blind regions which are within upper and lower limits of PRF for said radar system using the equation (PRF) 2RpF/cn + or - Delta (PRF) where n 1,2,3 . . . ,c is the speed of light, and Delta (PRF) is a predetermined value included to allow for half widths of velocity blind regions resulting from lack of target visibility in doppler shift for said predicted radial velocity, Rp, and said selected transmitting frequenCy, F, a different velocity blind region resulting for each value of n, computer means for calculating the highest PRF within the highest of said bands of PRF''s which places said selected target outside of range blind zones at ranges between Rn and Rn+ delta , where delta , a constant known width of each blind zone, is a function of radar receiver dead time which commences concurrently with each radar pulse transmission, and Rn is the range to the nth blind zone calculated by said computer means from the equation Rn cn/2(PRF) where n 1,2,3 . . . , and c is the speed of light, signal generating means for generating a transmitting signal at said selected transmitting frequency, and radar synchronizing means for timing transmission of radar pulses at said pulse repetition frequency by said transmitter, each pulse consisting of electromagnetic energy at said selected transmit frequency.
 10. Apparatus as defined in claim 9 including means for selecting said frequency, F, from one of a plurality of predetermined frequencies comprising means for sensing the magnitude of electromagnetic energy received by said receiver in a plurality of frequency channels, one channel for each of said predetermined frequencies, means for converting said magnitude of energy in each channel to a value in digital form, computational means for sorting said values in digital form in order of magnitude, and computer means for selecting said transmitting frequency, F, to be that frequency of a channel having the lowest of said values of electromagnetic energy sensed.
 11. Apparatus as defined in claim 10 wherein said signal generating means comprises means for generating a plurality of signals, one for each of said predetermined frequencies, and means for gating out to said transmitter the one transmitting signal at said selected frequency.
 12. Apparatus as defined in claim 11 wherein said radar synchronizing means comprises a source of clock pulses, and a counter for counting a predetermined number of clock pulses directly proportional to the reciprocal of PRF to time transmission of said radar pulses during a tracking swell, one radar pulse for each predetermined number of clock pulses. 